Hydraulic fluid circuits may use actuators to regulate the flow of hydraulic fluid to a hydraulic consumer, such as a hydraulic cylinder or a hydraulic motor. For example, hydraulic circuits may control the flow of hydraulic fluid to and from a hydraulic cylinder that extends and retracts to raise and lower a machine implement, such as a blade or bucket. The hydraulic fluid circuit may include a main spool actuation valve that is displaced to different positions in which the flow of the hydraulic fluid to the cylinder is actuated to extend or retract the hydraulic cylinder.
In electro-hydraulic circuits, actuation of the displacement of the main spool valve may be controlled by a pilot control system that includes smaller, electronically-controlled pressure relief valves (ePRVs) that apply hydraulic fluid pressure on the main spool actuation valve to achieve smooth modulation of main spool valve displacement to the desired position. The ePRVs may receive electrical current commands from an electronic control module (ECM) when a user inputs a command to raise or lower the implement, and the ePRVs may respond to the electrical current command by applying hydraulic fluid pressure on the main spool valve to cause the main spool valve to shift to the desired position. A first ePRV (when actuated) may apply hydraulic fluid pressure on a first side of the main spool valve via a first pilot line to displace the main spool valve to a first position, while a second ePRV may operate in a ‘relieving stage’ in which hydraulic fluid is permitted to drain through a second pilot line and the second ePRV to a tank as the main spool valve is displaced to the first position. Alternatively, the second ePRV (when actuated) may apply hydraulic fluid pressure on a second side of the main spool via the second pilot line to displace the main spool valve to a second position, while the first ePRV may operate in the relieving stage to allow hydraulic fluid to drain through the first pilot line and the first ePRV to the tank as the main spool is displaced to the second position.
While effective, the ePRVs used in current electro-hydraulic circuits may be designed with little resistance in the relieving stage so that the maximum hydraulic fluid pressure to achieve full displacement of the main spool valve is minimized. As a result, the pilot line of the ePRV operating in the relieving stage may have low fluid pressures. This low pressure condition on the ‘drain side’ of the main spool valve may allow dissolved air in the hydraulic fluid to come out of solution and form air bubbles in the hydraulic fluid. Such aeration of the hydraulic fluid may lower the bulk modulus of the hydraulic fluid, thereby creating a ‘spongy’ fluid condition on the drain side of the main spool valve. The increased ‘sponginess’ of the hydraulic fluid may result in variable resistance to the dynamic motion of the main spool valve as it is displaced to the desired position, allowing the main spool valve to oscillate or overshoot its desired position. This may ultimately result in poor controllability over the positioning of the implement.
U.S. Patent Application Publication Number 2013/0298542 discloses a hydraulic system having a control valve that shifts between three positions to control the flow of hydraulic fluid to a hydraulic cylinder. A return line downstream of the three-position control valve includes an electronically-controlled counter-pressure valve that varies the back pressure in the return line depending on the operation conditions. However, the reference does mention a pilot control system for the control valve, and it does not address the problem of aeration of the hydraulic fluid on the pilot control circuit of the main spool valve in such systems.
Accordingly, there is a need for improved designs for hydraulic fluid circuits that include a pilot control system for actuating displacement of a main spool valve. In such systems, there is a need for hydraulic fluid circuit designs that improve controllability over the displacement of the main spool valve.